Four-wheel drive (FWD) systems for vehicles can provide increased traction for the vehicle, particularly during inclement weather conditions, or off highway conditions, as compared to that provided by conventional two-wheel drive systems. Four-wheel drive systems typically include a torque transfer case with an input shaft connected to and driven by an output shaft of a transmission, a rear output shaft connected to drive rear wheels of the vehicle, a front output shaft connected to drive front wheels of the vehicle, and a clutch mechanism for drivingly connecting the input shaft to the front and rear output shafts.
Known four-wheel drive systems can operate in either a two-wheel drive mode, a four-wheel drive mode, or an “on-demand” drive mode. Known four-wheel drive systems can provide a direct drive connection between front and rear output shafts of the transfer case when the vehicle is operated in a four-wheel drive mode. A direct drive connection does not accommodate different front and rear wheel speeds, which can occur while turning the vehicle, thus limiting this four-wheel mode of operation to a “part time” basis to address lower friction road surface conditions, such as wet or snow covered pavement, where increased traction capability is desired. The “on-demand” drive mode is provided by a clutch assembly interactively associated with an electronic control system and a sensor arrangement. When sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque “on-demand” to the non-driven wheels. The amount of torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics as detected by the sensor arrangement. Four-wheel drive clutches require relatively high torque and are typically actuated using either a rotary electric motor and gear reduction system, or a plurality of pilot clutches. This type of known transfer case can be seen in U.S. Pat. No. 5,503,602; U.S. Pat. No. 5,465,820; and U.S. Pat. No. 5,462,496.
Known torque transfer cases can be equipped with a gear reduction and a synchronized range shift mechanism to permit “on-the-move” shifting between high-range and low-range drive modes. The synchronized range shift mechanism permits the vehicle operator to shift the transfer case between the high-range and low-range drive modes without stopping the vehicle. These known transfer cases can be seen in U.S. Pat. No. 7,294,086; U.S. Pat. No. 7,201,266; U.S. Pat. No. 7,059,462; U.S. Pat. No. 7,021,445; U.S. Pat. No. 6,997,299; U.S. Pat. No. 6,554,731; U.S. Pat. No. 6,458,056; U.S. Pat. No. 6,398,688; U.S. Pat. No. 6,354,977; U.S. Pat. No. 6,283,887; and U.S. Pat. No. 6,022,289.
Haldex Generation V is a commercially available electronically controllable all-wheel drive coupling or torque transfer case for motor vehicles. The Haldex Generation V includes an electronic control unit with vehicle dynamics software that can be customized to meet each vehicle maker's particular desires in terms of driving characteristics. The front and rear axle of the vehicle is connected via a wet multi-plate clutch which makes it possible to vary the torque distribution between the two axles. As the function of the Haldex Generation V is independent of the differential speed between the front and rear axle, full locking torque, if needed, is available at any given time and speed. When starting the vehicle, an electrical pump in the Haldex Generation V is started, swiftly providing the system with pressurized oil and thereby making the system ready for operation. The pump sets the pressure to a piston which in turn compresses a disc package of the wet multi-plate clutch. The level of pressure set depends on the torque level needed and on the driving situation. In traction/high slip conditions, a high pressure is delivered. In tight curves (i.e. parking) or at high speeds, a much lower pressure is provided.
Integrated into the housing of the Haldex all-wheel drive coupling, the electronic control unit (ECU) has been developed, designed and produced to endure the harsh environments to which the drivetrain is constantly subjected, including vibrations and thermal variations. The electronic control unit manages a valve which directs the torque transfer characteristics, ranging from a fully open position to a fully closed position. The open position is used during antilock braking system (ABS) operation and when stability control systems are activated. The Haldex all wheel drive (AWD) coupling opens in less the 60 milliseconds (ms). The fully closed position is used when accelerating and driving on soft ground. The software program determines the control of the coupling. The software program can include two main parts: a base software program and an application software program. The base software program can control the internal functions of the Haldex AWD coupling. One example is compensating for the variations in the viscosity of the oil due to operating temperature. The application software program can communicate with other active systems in the vehicle via a data bus. It determines the vehicle state with input signals from engine torque, engine revolutions, and wheel speeds. From this estimated vehicle state, the optimum torque distribution is determined in each condition. If any input signal is lost due to a failure in another ECU, the Haldex AWD coupling detects this and enters a limp home mode, maintaining best possible AWD function with the remaining signals. Only signal errors jeopardizing vehicle safety will result in a complete loss of AWD function (primarily front wheel drive (FWD) vehicle assumed). Every vehicle has its own requirement specification profile. This specification leads to different calibrations of the Haldex AWD coupling. A multiple set of parameters are available in the software itself. Online, the application software program can determine which set to use in the actual vehicle (e.g., family vs. sports sedan).